1. Field of the Invention
The present invention relates to an improvement in a starter for starting a high pressure discharge lamp such as a high pressure sodium lamp.
2. Description of the Prior Art
In order to facilitate the starting of a high pressure discharge lamp such as the high pressure sodium lamp that has a high starting voltage and that is difficult to start with the ordinary commercial line voltage, there is already known a starting circuit consisting of a serial connection of a resistor, for example an incandescent filament, and a bimetal switch, and connected parallel to an arc tube of the high pressure discharge lamp which is connected to an AC power source through an inductive stabilizer such as a choke coil. The high pressure discharge lamp incorporating such starting circuit together with the arc tube in an outer bulb is becoming rapidly popular, since such lamp can be used in combination with the existing stabilizer without the use of an independent starting device.
The function of such starting circuit, as already disclosed in the Japanese patent application Laid-Open No. 51-67174, will be briefly explained in the following in order to facilitate the understanding of the present invention.
Referring to FIG. 1, upon start of power supply from an AC power source 2, a current supplied through a stabilizer 1 to a starting circuit 6 heats a bimetal switch 5, thus opening said switch. The sudden current interruption by the opening of said bimetal switch 5 induces a high pulse voltage in the stabilizer 1 by the inverse electromotive force. With the heating current being cut off by said opening, the bimetal switch 5 is subsequently closed again to repeat the above-mentioned procedure. The pulses of an appropriate number thus generated by the repeated opening and closing of the bimetal switch 5 are applied to an arc tube 3 together with the power supply voltage to start the high pressure discharge lamp. As the operation of the discharge lamp becomes stabilized after its start, the arc tube increases the radiating heat to maintain said bimetal switch 5 in the open state. In the above-explained circuit, a current-limiting resistor 4 is inserted in the starting circuit 6 for the following purposes.
Firstly, without said resistor 4, the current in the starting circuit 6 will become as almost large as the shortcircuiting current of the stabilizer 1 to induce an extremely high pulse voltage exceeding several kilovolts when the bimetal switch 5 is opened, thus destroying the insulation of the stabilizer or other circuits.
Secondly, even if the discharge in the arc tube is successfully induced by one of plural pulse voltages generated by the repeated opening and closing of the bimetal switch 5, the discharge will be extinguished by the diversion of the discharge current to the starting circuit upon re-closing of the bimetal switch 5 if said resistor is not present in the starting circuit.
Thus, in such known circuit, the resistor 4 in the starting circuit 6 is appropriately selected so as to regulate the pulse voltage and to prevent the undesirable extinguishing of the arc tube.
Such known starting system is, however, unstable when powered by a commercial AC power supply because of the fluctuation in the instantaneous current at the opening of the bimetal switch, in the opening speed thereof and in the arc state between the contacts of the bimetal switch. As an example, FIG. 2 shows the distribution of peak pulse voltages obtained in 150 repeated operations of the bimetal switch 5 in a circuit shown in FIG. 1, employing an AC power source 2 of 200 V; a stabilizer choke coil 1 of ca. 120 mH; a high pressure sodium arc tube 3 of 360 W; and an incandescent filament resistor 4 with an operating resistance of ca. 200 .OMEGA.. As shown in FIG. 2, the pulse voltages show considerable fluctuation ranging from ca. 1 KV to ca. 5 KV. In such known system, a certain fluctuation in the pulse voltage is unavoidable in order to ensure satisfactory starting performance, thus leading occasionally to the appearance of a very high pulse voltage. Although such high pulse voltage can be avoided by an appropriate adjustment of the resistance of said resistor 4 in the starting circuit 6 to displace said voltage distribution toward a lower value, such adjustment will inevitably lower the starting probability of the discharge lamp. Stated otherwise, such known system is inevitably associated with the appearance of unnecessarily high pulse voltages in order to maintain a certain desired level of starting probability, and various methods have been proposed to eliminate such pulse voltages.
As an example, the Japanese patent ppplication Laid-Open No. 53-103668 proposes, for absorbing unnecessarily high pulse voltage, to connect a pulse-absorbing discharge tube starting at a voltage higher than the starting voltage of the arc tube, parallel to said arc tube or to the bimetal switch in the starting circuit. In practice, however, it is difficult to find an appropriate discharge tube capable of absorbing the pulse voltage in response to a pulse width as short as ca. 1 .mu.sec. generated by the function of the bimetal switch in the starting circuit, and such method is found to be unable to resolve the unstable function resulting from the fluctuation of the pulse voltage. As a method for eliminating the fluctuation in the pulse voltage, the Japanese patent application Laid-Open No. 55-143771 proposes to enclose a gas for reducing the pulse voltage generated by the function of the bimetal switch 5 in the outer bulb 7 incorporating the starting circuit 6 shown in FIG. 1. This method is to suppress the generation of unnecessarily high voltages, and it is confirmed that the generated pulse voltage, with reduced high frequency components and increased pulse duration, exhibits stable starting performance despite the generally reduced peak voltage. However such method is still defective in that a predetermined amount of gas has to be enclosed in the outer bulb during manufacture, and in that it is difficult to maintain stable performance as gaseous impurities are emitted from the arc tube and the support structure thereof during use.